Clean and slice the mushrooms. Heat the oil or butter in a large pan (which has a lid) and fry the mushrooms for about 5 minutes or until golden brown. Remove to a plate and set aside.
Finely chop the onions and the garlic. Place them in the same pan together with the ground beef, and cook for about 4-5 minutes until the meat is brown and the onions slightly golden. Break the lumps of meat using a wooden spoon during this time.
Whisk together the flour and the water. Add the mixture to the pan together with the chicken or beef stock, tomato paste, Worcestershire sauce, salt, and pepper. Mix well.
Turn the heat down to low, cover the pan, and simmer for 1 hour, stirring from time to time. Add the mushrooms to the sauce. Continue simmering for 10 more minutes without the lid; the sauce should thicken slightly.
Stir in the sour cream or smetana. Don't let the dish come to a boil again.
Sprinkle some chopped parsley over the ground beef stroganoff and serve with noodles, rice, or mashed potatoes.
